"You climb a mountain situated close to the sea or a level plain, and then observe the setting of the sun and find out the dip of the horizon... [Then] find the value of the perpendicular of the mountain. You multiply this height into the sine of the complementary angle of the dip, and divide the total by the versed sine of this dip itself. Then multiply (twice) the quotient into 22 and divide the result by 7. You will get the... earth's circumference (in the same units) in which the height of the mountain has been found."